Thermal mass flow sensors or, synonymously, caloric mass flow sensors are based on the principle that heat is supplied to a flowing fluid. The heat transfer functions between the sensor and the fluid are thereby measured. The known 2-element principle has two elements which are arranged one behind the other in the flow direction and perform both the heating function and the function of temperature measurement. They are mounted on a heat-conducting line through which fluid flows, they are heated electrically and they are cooled by the flowing fluid. If both elements are heated with the same output, the line exhibits a symmetrical temperature profile around the elements when the fluid is at rest, that is to say the temperature difference between the elements is theoretically zero (see broken line in FIG. 1). When the fluid is flowing through the line, on the other hand, the temperature profile is displaced in the flow direction. A temperature difference ΔT of the elements which is proportional to the mass flow is thereby measured (see solid line in FIG. 1). In the case of the 3-element principle, the functions of heating and temperature measurement are separate. One or more heating elements are thereby arranged as centrally as possible between two temperature sensors located upstream and downstream. The measured temperature difference is again a measure of the mass flow.
In thermal mass flow sensors, the actual relationship between the mass flow and the measured temperature difference is complex. Determining factors are in particular the structural form (spacing of the sensors, size and shape of the heat exchange surfaces, axial and radial thermal resistivities, contact resistances), the flow conditions of the fluid, the fluid properties (viscosity, thermal conductivity, specific heat capacity), the installation position of the sensor and the ambient conditions. The functional relationship between the measured temperature difference and the mass flow is therefore determined empirically by multipoint calibration and stored in the form of sensor-specific characteristic curves. A large number of thermal mass flow sensors are known, in which the error sources of the measurement principle are limited or eliminated by different technical means.
U.S. Pat. No. 4,517,838 A, U.S. Pat. No. 5,347,861 A, U.S. Pat. No. 5,373,737 A, and EP 0 395 126 A1 describe sensors in which the measurement takes place, in a U-shaped bypass to the line in which the fluid is flowing. U.S. Pat. No. 4,517,838 A, U.S. Pat. No. 5,347,861 A and U.S. Pat. No. 5,373,737 A describe sensors according to the 2-element principle. In U.S. Pat. No. 4,517,838 A, the sensor pipe is surrounded by a narrow gap, as a result of which the effect of the installation position is reduced and the time constant is shortened. U.S. Pat. No. 5,347,861 A achieves the same aims by means of a heated thermal bridge over the sensor pipe. U.S. Pat. No. 5,373,737 A discloses an active cooling plate for eliminating the influence of the ambient temperature. In EP 0 395 126 A1, on the other hand, the 3-element principle is used. The start and end temperatures of the bypass are kept the same by strong thermal coupling; in order to compensate for the null drift, a two-part heating element is used. The measurement of the temperature difference takes place in such a manner that thermocouples or thermopiles connect the temperature measuring positions upstream and downstream of the heating element directly.
U.S. Pat. No. 7,895,888 B2 describes heater and temperature sensor chips which are secured to the surface of small pipelines and which operate according to the 3-element principle. In order to expand the measuring range, a plurality of temperature sensor pairs are arranged symmetrically to the central heater chip at different distances therefrom.
EP 0 137 687 A1, DE 43 24 040 A1, U.S. Pat. No. 7,197,953 B2 and WO 2007/063407 A2 describe sensors according to the 3-element principle which are produced by means of silicon technology and are integrated or inserted into flow channels. In EP 0 137 687 A1, the measurement is carried out in one or in a plurality of bypasses. In order to compensate for the temperature dependency of the characteristic curve, DE 43 24 040 A1 uses additional heater and media temperature sensors, the heater temperature being regulated to be constant with a changing mass flow and the media temperature being tracked via a characteristic curve which is dependent on material values. In U.S. Pat. No. 7,197,953 B2, Pt thin-film sensors for temperature measurement in combination with specific correlations for improving the accuracy of measurement are documented. WO 2007/063407 A2 describes the purposive distribution of the heat energy through heat-conducting material in order to reduce the systematic effects due to relatively low temperature differences. WO 01/14839 A1 describes a sensor whose heating element is operated in a pulsed manner. The mass flow is determined from the progression of the heating and cooling process at the sensor over time.
DE 689 03 678 T2 discloses a device for measuring the flow in liquids. A heating element is thereby arranged in a first block in order to increase the temperature thereof relative to a second block, a pipe being provided with a metal foil over its entire length. The metal foil and the pipe ensure that supplied heat energy flows from the first block to the second block, so that the whole region functions as a heat exchanger. This results in a temperature profile of the heat exchanger which is linear in the flow direction, the temperature of the heat exchanger increasing upstream in the flow direction.
U.S. Pat. No. 4,817,427 A discloses a device for measuring the flow of water in plant stems. Energy supplied via a main heater dissipates in the form of four different heat flows. In order to keep the sum of three of the four heat flows constant without having to determine their individual values exactly, additional heaters are used with varying water flows, regulation of which is effected via temperature gradients in the respective sections, which are detected by thermocouples. A homogenisation of the surface temperature of the main heater is achieved via copper foils.
In Thermal mass-flow meter, J. Phys. E 21, 1988, p. 994-997, J. H. Huij sing et al. describe a device for the thermal measurement of the mass flow rate, which device has three copper blocks which are arranged over the flow cross-section, the fluid flowing into the copper blocks through holes. The supply of a heat output takes place via the middle copper block. This arrangement ensures that the heat output is distributed evenly over the flow cross-section. The measurement principle is based on the temperature change of the fluid stream by the supplied heat output, the temperature profile of the heater in the flow direction playing no part.